Re-equilibration processes in fluid inclusion assemblages

Abstract

Post-entrapment modifications reduce the reliability of fluid inclusions to determine trapping conditions in rock. Processes that may modify fluid inclusion properties are experimentally identified in this study using synthetic fluid inclusions in quartz with a well-defined composition and density. Modifications are characterized with microthermometry (homogenization and dissolution temperatures) and Raman-spectroscopy in binary fluid systems H2O-D2O and H2O-NaCl. Three distinct processes were identified in this study: (1) diffusion of H2O and D2O; (2) crystal-recovery, expulsion of H2O and accumulation of quartz in inclusions (preferential H2O loss); (3) irreversible total volume increase at the α-β quartz transition. Diffusion is caused by H2O fugacity gradients and can be modelled according to classical diffusion models. The variability of re-equilibrated properties in fluid inclusion assemblages depends on time, temperature, diffusion distance and the size of fluid inclusions. Negative pressure gradients (internal under-pressure) induce the crystal-recovery process, in which H2O is preferentially extracted from inclusions that simultaneously shrink by the inward growth of quartz. This process reduces the H2O concentration and increases the fluid density by total volume loss. Temperature and time are also controlling factors of this process, which is able to transport H2O against fugacity gradients.